Study on PCM-based Cylindrical Battery Thermal Management System Towards Improvement On Cooling Performance and Fire Safety

Abstract

Given the sensitivity of lithium-ion batteries (LIBs) to operating temperature, which may result in battery fires, it is necessary to enhance LIB safety by developing reliable and safe battery thermal management systems (BTMs). Phase change material (PCM)-based BTMs are promising solution, but currently used organic PCMs have several inherent deficiencies, such as flammability and inadequate secondary heat dissipation. Moreover, the understanding on the degradation of PCM cooling and the effect of PCMs on LIB fires remain very limited, hindering further application of PCM-based BTMs for LIBs. The work described in thesis aims to clarify the PCM cooling mechanism, identify the thermal and fire hazards associated with PCM-based BTMs, and recommend methods for enhancing the fire safety of PCM-based BTMs. The main body of this work consists of four parts, as described below.

In part one, the PCM cooling mechanism and its influencing factors are investigated. The dynamic thermal balance model of an LIB-PCM is established, and the thermal behaviors of batteries with and without a PCM, respectively, are explored. Subsequently, the effects of several parameters—such as the phase-change temperature (PCT), rest time during charge–discharge cycles, and current mode—on PCM cooling performance are examined. The findings are expected to provide guidance for the utilization of PCM cooling technologies and design of PCM-based LIB modules.

In part two, the degradation of PCM cooling behavior under high temperatures is investigated. The influence of PCMs on the heat dissipation behavior of LIBs at various ambient temperatures and with various contents of thermally conductive additives is studied. In addition, PCMs are coupled with active liquid cooling to enhance the secondary heat-dissipation performance of PCM-based BTMs.

In part three, the fire hazards associated with PCM-based BTMs and the burning characteristics of PCMs are explored. A series of experiments is performed at the material and LIB module levels. A PCM fire-spread model is established by examining infrared video images, LIB surface temperatures, and results of material characterization before and after combustion. The fire dynamics and mechanism whereby PCMs affect the thermal runaway propagation of LIB modules are clarified.

In part four, the ability of a PCM-coupled thermal insulation design to inhibit LIB thermal runaway propagation is investigated. Considering the fire risk of PCM-based BTMs identified in part three, a composite-PCM-based BTM is constructed to optimize heat dissipation and inhibit runaway thermal propagation. Through a series of experiments, the mechanism whereby a thermal insulation mitigates runaway thermal propagation in LIB modules is illustrated, and composite BTMs are designed to enhance the fire safety of PCM-based BTMs.

The findings described in this thesis deepen understanding on the occurrence and evolution processes of thermal hazards in PCM-based BTMs. Moreover, the findings show that appropriate design of thermal protection strategies can delay or mitigate thermal hazards, which has important theoretical and practical implications for reducing the fire accident rate of electric vehicles, increasing the escape time of occupants and improving fire rescue.

Date of Award	19 Jul 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jian Wang (External Supervisor) & Kwok Kit Richard YUEN (Supervisor)